For early diagnosis of lifestyle-related diseases or diseases ranking as leading causes of loss of eyesight, eye examination is widely performed. An eye tomographic imaging apparatus such as an optical coherence tomography (OCT) enables three-dimensional observation of an internal state of a retinal layer, and is therefore expected to be useful for more accurate diagnosis of the diseases.
FIG. 1 illustrates a schematic diagram of a tomographic image of a macula lutea portion of a retina taken by the OCT. In FIG. 1, “F” represents a fundus image, “RXY” represents an imaging range on the fundus, “MC” represents a center of the macula lutea, “T1” to “Tn” represent two-dimensional tomographic images constituting a volume image (B-scan image, hereinafter referred to as tomographic image), and “A” represents a scan line (A-scan line) in a depth direction of the retina (“z” direction). Further, the tomographic image Tn shows an internal limiting membrane 1, a nerve fiber layer 2, a ganglion cell layer 3, an inner plexiform layer 4, an inner nuclear layer 5, an outer plexiform layer 6, an outer nuclear layer 7, an external limiting membrane 8, a photoreceptor inner/outer segment junction 9, and a retinal pigment epithelium boundary 10. When this tomographic image is input, for example, in a case where a thickness between the internal limiting membrane 1 and the retinal pigment epithelium boundary 10, that is, a thickness of the retina, can be measured, this can be useful for diagnosis of various diseases in which a visual disorder is caused by a variation of the thickness of the retina. This imaging method of acquiring a volume image by taking images of a two-dimensional range on the fundus is referred to as three-dimensional (3D) scan.
In addition, the OCT imaging uses an imaging method of repeatedly imaging an irradiated area on the same line of the fundus, and calculating an arithmetic mean of the obtained tomographic images so as to output a high-definition tomographic image with little noise. Hereinafter, this method is referred to as line scan. With this imaging method, it is possible to closely observe an anatomical structure inside the retinal layer at the center of the macula lutea or in a lesion region, which is an important region for diagnosis of fundus diseases. In the imaging of the line scan, in addition to a single tomographic image, multiple tomographic images may be taken by setting multiple lines as an imaging area on the fundus.
In this way, on a clinical site, two types of imaging methods are used in combination in many cases, that is, the 3D scan for acquiring a wide-range volume image so as to prevent unintended omission of a tomographic image, and the line scan for acquiring a high-definition image for detailed observation of the lesion.
In this case, in the line scan, only the tomographic image corresponding to a specific line area on the fundus is acquired. Therefore, it is necessary to appropriately set an imaging position so that the anatomical structures of the center of the macula lutea and the lesion region are included in the tomographic image. However, in the conventional OCT apparatus, it has been necessary for an operator to manually set the imaging position, and hence much time and effort has been required to search for the center of the macula lutea and the lesion position on the fundus. In addition, there has been a risk that an appropriate position cannot be imaged because the operator overlooks the lesion or erroneously sets a position shifted from the center of the macula lutea as the imaging position.
To address this problem, Patent Literature 1 discloses a technology in which a position of a characteristic region is identified in a tomographic image acquired through preliminary measurement, and then an irradiation position of a signal light is changed based on the position of the characteristic region so that the characteristic region is rendered at a predetermined position in a frame through main measurement. Specifically, a position of the center of the macula lutea or a dent indicating a center of an optic nerve mamilla is detected as the characteristic region in the tomographic image through the preliminary measurement, and hence the irradiation position is determined so that the position is rendered at the predetermined position in the frame through the main measurement.
In addition, Patent Literature 2 discloses a technology of detecting a candidate lesion in a wide-range image of the fundus, and determining a spatial range of a tomographic image to be taken, a scan line interval, and a scan order and direction based on a type and a range of the detected candidate lesion. Further, there is also disclosed a technology of determining whether or not it is necessary to acquire the tomographic image according to seriousness of the candidate lesion or a region in which the candidate lesion occurs.
However, the conventional technologies have the following problems. In the technology disclosed in Patent Literature 1 described above, in a case where the characteristic region has a structure extending in an isotropic manner from the center (dent), such as the macula lutea portion or the optic nerve mamilla portion, when only the center position is identified, the irradiation position can be determined so that the character is rendered in the tomographic image. However, in a case where the characteristic region is a lesion, the lesion does not necessarily have an isotropic extension from its center position. For instance, in a case of a macula lutea edema observed in a diabetic retinopathy, blood flows out of a capillary aneurysm or a blood vessel so that a low brightness area called a cyst occurs in the retina, and hence the retina swells. In this case, the retina swells not in an isotropic manner from the center position of the lesion, but in various different manners according to a position or a degree of the generated cyst. Therefore, when the imaging position is automatically set in the method described in Patent Literature 1, the acquired tomographic image may include a part of the lesion but does not necessarily have the above-mentioned feature of the macula lutea edema rendered therein. Therefore, this is not necessarily an appropriate tomographic image for detailed observation of the lesion.
In addition, in the technology disclosed in Patent Literature 2, in a case where the 3D scan image is taken at close scan intervals, when the spatial range of the tomographic image is determined according to extension of the lesion, a tomographic image useful for diagnosis of a lesion can be included in the acquired volume image. However, in a case where multiple line scan images which assume the use of the arithmetic mean of the tomographic images as described above are taken, the tomographic images are taken at separate scan intervals, and therefore do not necessarily include the tomographic image capturing the above-mentioned feature of the lesion.